Gartner helped arrange 120 buses of abortion protesters from southwestern Pennsylvania to go to the annual March for Life in Washington.

On the other side of the issue, the National Organization for Women will hold a rally and vigil in front of the Supreme Court to "make the point that abortion care is health care," says NOW president Terry O'Neill.

Some activities will be online, such as a new event called the Virtual March for Life, organized by Americans United for Life Action, and the fifth annual Blog for Choice Day spearheaded by NARAL Pro-Choice America.

By Thursday evening, more than 56,000 people had signed up for the virtual march, which allows people who can't travel to Washington "an opportunity to be heard on the issue and express solidarity with the people who are marching in person," says Charmaine Yoest, president of Americans United For Life Action.

"We're hearing from people all over the country who are saying, 'We've always wanted to participate and this gives me a way,' " she says.

For Blog for Choice Day, bloggers are asked to describe what the phrase "trust women" means to them, says Ted Miller, spokesman for NARAL Pro-Choice America.

"We know that not everyone can lobby a member of Congress in Washington," he says. This gives participants a way "to explain why they're pro-choice in their own words."

Although prospects for passage of a health care legislation were thrown into question with the election of Republican Scott Brown in Massachusetts on Tuesday, groups on both sides agree that it will be the hot topic at Friday's events.

Both sides find fault with Senate and House versions of the legislation. Abortion opponents say they fear proposals would fund an expansion of abortion, Yoest says. Abortion rights supporters fear funding restrictions will limit coverage of abortion, O'Neill says.

Adam Sonfield, senior public policy associate for the Guttmacher Institute, a think tank that studies sexual and reproductive health, says that if health care legislation eventually does pass, there will probably be fewer abortions.

If more people have health insurance, "there will be more coverage of family planning services," he says, "which means more women will have the care they need to help them prevent unplanned pregnancies."

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